Metal nanoparticles are ultrafine particles 1 to 100 nm in diameter, which are known to fuse together spontaneously due to the extreme instability of the atoms on the particle surface, forming coarser particles. It is therefor normal to stabilize metal nanoparticles by covering the surfaces with organic protective groups. Unlike bulk metal, metal nanoparticles exhibit the characteristic properties of low melting point and low-temperature sintering, and are used in conductive pastes for forming wiring in engineering applications.
Metal nanoparticles are often classified according to the method of synthesis. Methods of metal nanoparticle synthesis are classified generally into two types: physical methods in which bulk metal is pulverized to obtain nanoparticles, and chemical methods in which zero-valent metal atoms are produced from a metal salt, metal complex or other precursor, and then aggregated to obtain nanoparticles. One physical method is pulverization, in which a ball mill or other device is used to grind metal down into smaller pieces, thereby obtaining metal nanoparticles. However, the particles obtained by this method have a broad particle size distribution, and it is difficult to obtain particles hundreds of nanometers or less in size. On the other hand, chemical methods include 1) the laser synthesis method, in which metal nanoparticles are synthesized by heating a reactive gas with a CO2 laser, 2) the spray pyrolysis method, in which metal nanoparticles are obtained by spraying a metal salt solution in a high-temperature atmosphere, causing the instantaneous evaporation and pyrolysis of the solution, and 3) the reduction method, in which metal nanoparticles are obtained by a reduction reaction from a metal salt solution, but none of these methods are suited to quantity synthesis.
To resolve these problems of existing metal nanoparticle synthesis methods, the inventors in this case developed a thermal decomposition control method whereby a metal nanoparticle can be synthesized simply by heating a metal complex as the metal source in the absence of a solvent (Patent Document 1, Patent Document 2, etc.). The primary feature of this thermal decomposition control method is the simplicity of heating in the absence of a solvent, which allows for quantity synthesis. It has also been found that adding an organic compound or the like with a mild reducing character to the reaction system serves to moderate the reaction conditions, and design of the particle diameter, shape and surface protective layer is also possible.
Metal nanoparticles are being actively studied for industrial application in a variety of fields, including microwiring technologies using metal nanoparticles. Because the surfaces of metal nanoparticles are covered with an organic protective layer, they are highly solvent-dispersible, and wiring at lower temperatures than before is anticipated using the characteristic low-temperature fusion property of nanoparticles. At present, most applications involve wiring materials using silver nanoparticles, but silver is rare and therefore expensive, and it is also considered problematic because when used under conditions of high humidity it is extremely liable to a phenomenon called migration, in which the silver ionizes and is re-deposited outside the circuits, causing short-circuits between electrodes. Attention is therefore shifting to copper nanoparticles, which are expected to be cheaper and to cause little or no migration.
The problem with copper is that it readily oxidizes in air. In fact, synthesis of copper particles has already been studied by a variety of methods (Patent Document 3, Patent Document 4, etc.), but no technique has focused on the problem of oxidation, and no technology has been proposed for solving the problem of oxidation.    Patent Document 1: Japanese Patent Application Publication No. 2007-63579    Patent Document 2: Japanese Patent Application Publication No. 2007-63580    Patent Document 3: Japanese Patent Application Publication No. 2008-19503    Patent Document 4: Japanese Patent Application Publication No. 2008-95195